Many implantable medical devices (IMDs) that are used to treat or monitor patients suffering from a variety of conditions rely on electrochemical cells for providing the energy needed to power the device electronics and generate therapeutic electrical stimulation pulses. Examples of such IMDs include implantable pacemakers and implantable cardioverter-defibrillators (ICDs), which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers as necessary. Pacemakers deliver relatively low-voltage pacing pulses in one or more heart chambers. ICDs can deliver high-voltage cardioversion and defibrillation shocks in addition to low-voltage pacing pulses
IMDs including pacemakers, ICDs, drug pumps, neurostimulators, physiological monitors such as hemodynamic monitors or ECG monitors, typically require at least one battery to power the various components and circuitry used for performing the device functions. Pacemakers and ICDs generally include pulse generating circuitry required for delivering pacing and/or cardioversion and defibrillation pulses, control circuitry, telemetry circuitry, recharge circuitry and other circuitry that require an energy source. In addition to a battery, ICDs include at least one high-voltage capacitor for use in generating high-voltage cardioversion and defibrillation pulses.
IMDs are preferably designed with a minimal size and mass to minimize patient discomfort and prevent tissue erosion at the implant site. Capacitors contribute substantially to the overall size and mass of an ICD. Capacitors for use in an ICD are typically provided with a hermetically-sealed encasement for housing an electrode assembly, including an anode and cathode, an electrolyte, and other components such as a separator, electrode connector feedthroughs and lead wires. The encasement includes a case and a cover that are sealed through joining processes such as laser welding after assembling the cell components within the case.
Factors affecting the performance of electrolytic capacitors include the effective surface area of the anodes and cathodes that can be contacted by the electrolyte, the dielectric constant of the oxide formed on the electrode surface, and the thickness and properties of the dielectric layer. To improve the capacitor cell performance, porous or surface-enhanced electrode substrate materials are used to effectively increase the electrode surface area. For example, flat electrolytic capacitors often include aluminum sheets that are etched or perforated to increase the electrode surface area. Pellet or slug-type electrodes are formed from a valve metal powder that is pressed and formed into a porous substrate.
Typically, an oxide dielectric layer is grown anodically upon exposed surfaces of the electrode when the electrode is immersed in a formation electrolyte. The composition of the oxide layer grown anodically is limited to oxides of the elements found in the electrode substrate. Deposition of a dielectric layer having a composition differing from the substrate composition may be deposited onto an electrode substrate, for example by physical vapor deposition. However, such deposition of a dielectric layer onto a highly structured substrate surface is not likely to achieve uniform coverage of the substrate surface, resulting in defects such as “pin holes”. Non-uniform coverage of the substrate surface will result in inferior electrode performance.